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We have grown untwinned YBa2Cu3O 7−δ (YBCO) films on (110) MgO substrates that were preannealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear, in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO. * riccardo.arpaia@chalmers.se † floriana.lombardi@chalmers.se duced by the substrate. In the case of YBa 2 Cu 3 O 7−δ (YBCO), this requires the growth of films which must be untwinned, to preserve the anisotropies related to the orthorhombicity of the unit cell, and at the same time compressively strained. The growth of untwinned films, where aligned CuO chains are present throughout the sample, is not an easy task since c-axis oriented YBCO films usually grow twinned on the substrates commonly used for the deposition of cuprates. This twinning, caused by a random exchange of the in-plane a and b axis, influences the electric and magnetic properties of the films [15][16][17][18]. The best untwinned YBCO films, reported so far, have been achieved on SrTiO 3 (STO) substrates with a vicinal miscut angle, i.e. with a small misorientation, with respect to the (001) plane direction [19,20]. However, the STO substrate induces a tensile stress into the YBCO films.Here we report on the growth of untwinned YBCO films under compressive strain. We succeeded in this task, by an in-situ thermal treatment of (110) oriented MgO substrates, prior to the deposition of the YBCO thin films. The thermal treatment results in a reconstruction of the substrate surface, favoring the growth of untwinned YBCO. Moreover, the film is subject to a very peculiar compressive strain, as a result of the large mismatch between the in-plane lattice parameters of the film and the substrate: the b axis shrinks, and tilts around the normal to the MgO surface.The manuscript is organized as follows: In Section II, we start with an overview of the past results on un-arXiv:1908.02637v2 [cond-mat.supr-con]

Inductance is a key parameter when optimizing the performance of superconducting quantum interference device (SQUID) magnetometers made from the high temperature superconductor YBa 2 Cu 3 O 7−x (YBCO) because lower SQUID inductance L leads to lower flux noise, but also weaker coupling to the pickup loop. In order to optimize the SQUID design, we combine inductance simulations and measurements to extract the different inductance contributions, and measure the dependence of the transfer function V Φ and flux noise SA comparison between two samples shows that the kinetic inductance contribution varies strongly with film quality, hence making inductance measurements a crucial part of the SQUID characterisation. Thanks to the improved estimation of the kinetic inductance contribution, previously found discrepancies between theoretical estimates and measured values of V Φ and S 1/2 Φ could to a large extent be avoided. We then use the measurements and improved theoretical estimations to optimize the SQUID geometry and reach a noise level of S 1/2 B = 44 fT/ √ Hz for the best SQUID magnetometer with a 8.6 mm × 9.2 mm directly coupled pickup loop. Lastly, we demonstrate a method for reliable one-time sensor calibration that is constant in a temperature range of several kelvin despite the presence of temperature dependent coupling contributions, such as the kinetic inductance. The found variability of the kinetic inductance contribution has implications not only for the design of YBCO SQUID magnetometers, but for all narrow linewidth SQUID-based devices operated close to their critical temperature.

The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO), hosts a quasi-two-dimensional electron gas (q2DEG), two-dimensional superconductivity, ferromagnetism, and giant Rashba spin-orbit coupling. The co-existence of two-dimensional superconductivity with gate-tunable spin-orbit coupling and multiband occupation is of particular interest for the realization of unconventional superconducting pairing. To investigate the symmetry of the superconducting order parameter, phase sensitive measurements of the Josephson effect are required. We describe an approach for the fabrication of artificial superconducting weak links at the LAO/STO interface using direct high-resolution electron beam lithography and low-energy argon ion beam irradiation. The method does not require lift-off steps or sacrificial layers. Therefore, resolution is only limited by the electron beam lithography and pattern transfer. We have realized superconducting weak links with a barrier thickness of 30–100 nm. The barrier transparency of the weak links can be controlled by the irradiation dose and further tuned by a gate voltage. Our results open up new possibilities for the realization of quantum devices in oxide interfaces.